A gate valve of the type referred to, whose housing has two aligned apertures forming part of the flow path to be selectively blocked, has been described in U.S. Pat. No. 2,825,528. According to that patent, a valve body forms a cylinder containing a hydraulic liquid and has a pair of disks removably secured to it by circular bellows so that they are outwardly displaceable by hydraulic pressure, against the force of biasing springs, into contact with respective seat rings surrounding the associated apertures. The valve body is engaged by a threaded stem which can be manually reciprocated, with the aid of a handwheel, to thrust that body into its blocking position through the intermediary of a compressible coil spring. When the valve body strikes a stop defining its blocking position, further rotation of the handwheel advances the stem to compress the coil spring while an extremity of the stem enters the cylinder to displace the hydraulic liquid therein which then drives the two disks into contact with the associated seat rings. When the valve is to be opened, the handwheel is rotated in reverse to retract the stem whereupon the biasing springs withdraw the disks from their seat rings; thereafter, further rotation of the handwheel retracts the disengaged valve body into an unblocking position away from the flow path. Naturally, the force exerted upon the stem during the closure stroke must be sufficient to overcome the reactions of the coil spring and of the biasing springs.
A considerably simpler arrangement, in which the valve body is a frustoconical plug and is rigid with its reciprocable stem, is known from German printed specification No. 1,058,802 published June 4, 1958. The plug is unitary and not provided with separately movable shutter members, being thus only able to control a flow in a transverse pipe provided with aligned holes of different diameters closely surrounding that plug in its inserted position.
From commonly owned German laid-open application No. 26 01 120, published July 28, 1977, it is further known to provide the body of a gate valve with two mechanically separable shutter members interconnected by a scissor linkage. A spreader reciprocable in the valve housing is coupled via a compression spring with an operating rod which during a closure stroke drives the valve body into a blocking position and thereupon actuates the spreader to let the scissor linkage force the two shutter members into contact with associated sealing rings.